1. Field of the Invention
The present invention relates generally to electrical power converters, and more particularly, to sensing and monitoring of electrical power in DC-to-DC switching-mode power converters.
2. Description of the Related Art
To convert one DC (Direct Current) level to another, a DC-to-DC switching-mode converter is commonly employed to perform the task. FIG. 1 shows a conventional DC-to-DC switching-mode converter signified by the reference numeral 2. The converter 2 has an input circuit 4 and an output circuit 6 separated by a transformer 8. The input circuit 4 includes a switch 10 controlled by a control circuit 12. One terminal of the switch 10 is tied to the primary winding 12 of the transformer 8. The other terminal of the switch 10 is connected to a DC input VIN. The output circuit 6 includes an inductor 15 and a capacitor 16 connected in series. The common connection of the inductor 15 and the capacitor 16 drives a load 18. The primary and secondary windings 12 and 20 of the transformer 8 have N1 and N2 winding turns, respectively. Disposed between the secondary winding 20 of the transformer 8 and the inductor 15 is a diode 14. Further, connected across the inductor 15 and the capacitor 16 combination is another diode 17.
During operation, an input DC voltage VIN is supplied to one terminal of the switch 10. The control circuit 12 generates a periodic output which in essence periodically turns on and off the switch 10. As a consequence, a time-varying current iP with periodic current pulses flows through the primary winding 12 of the transformer 8. In this specification, the lowercase alphabets are used to denote parameters that vary with time. Because the primary and secondary windings 12 and 20 are inductively coupled together, a secondary current iS is thereby induced in the secondary winding 20. The secondary current iS passes through the diode 14 which admits only positive current cycles but blocks away any negative counterparts. Since both the inductor 15 and the capacitor 16 respectively assume high inductive and capacitive values, they cooperatively contribute to a slow time-constant.
When the secondary current iS with a positive current cycle impinges upon the secondary circuit 6, the diode 14 is forward biased. The secondary current iS, after passing through the forward biased diode 14, charges sluggishly through the inductor 15 and capacitor 16. At this juncture, the power converter 2 is said to be in the forward rectification mode.
When the diode 14 is not forward biased, to maintain continuous current flow, magnetic energy stored in the inductor 15 discharges into the capacitor 16 and flows through the diode 17. The power converter 2 is then said to be in the freewheeling mode.
The alternating operating of the forward rectification mode and the freewheeling mode basically allows a DC voltage level to be maintained across the capacitor 16. The DC voltage level is utilized as the DC output voltage VOUT driving the load 18. Depending on the impedance of the load 18, a DC current IOUT is established passing through the load 18, in accordance with Ohm""s law.
In practice, the load current IOUT needs to be monitored. Insufficient current flowing through the load 18 may render the load 18 inoperative or malfunctional. On the other hand, excessive current IOUT feeding the load 18 may damage the load 18 and also the power converter 2. Different applications require different current monitoring schemes. For example, in some applications in which the load 18 may require over current protection and thus the upper limit of the output current IOUT must be detected and maintained. As another example, in a shared-load arrangement, the common current IOUT driving the shared load 18 needs also be ascertained for proper load current allocation. Furthermore, in usages where the instantaneous power needs to be known, the instantaneous value of the output current IOUT must also be instantaneously detected and reported.
Heretofore, monitoring of the output current IOUT has mostly been conducted on the secondary side of the transformer 8 by directly measuring the current path through the load 18. A common approach is to place a shunt resistor in series with the load 18. Another known approach is to couple a Hall effect device to the load 18.
First, the use of a Hall effect device involves complicated circuit design and thus costly. In addition, a Hall effect device is spacious. The use of Hall effect devices in most instances is not practical.
The use of shunt resistors for current detection is a common practice but it involves considerable drawbacks. To understand the problems associated with using a shunt resistor, the basic principles of a DC-to-DC converter needs first be explained. Reference is now directed back to FIG. 1. In the DC-to-DC converter 2, if the transformer 8 is a step-down transformer, as is known in the art, the primary and secondary voltages vP and vS, across the primary and secondary windings 12 and 20, respectively, assume a directly proportional relationship in accordance with the following algebraic expression:                                           v            P                                v            S                          =                  N1          N2                                    (        1        )            
However, the primary and secondary currents iP and iS relate to each other by an inversely proportional relationship as expressed by the following mathematical relationship:                                           i            P                                i            S                          =                  N2          N1                                    (        2        )            
In a step-down transformer, the secondary voltage vS is lower than the primary voltage vP. However, the secondary current iS is higher than the corresponding primary current iP. In most applications with a DC-to-DC converter, such as the converter 2, the output voltage VOUT is much lower than the input voltage VIN, resulting in the output current IOUT much higher than the corresponding input current IIN. In practice, sensing a high current always posses technical complications and sometimes fraught with danger. Chief among all is the difficulty in the power management of the shunt resistor. Even though the shunt resistor is normally designed to have a small ohmic value, in terms of degree of difficulty in managing the power of the shunt resistor, the high output current IOUT passing through the shunt resistor more than compensates for the choice of a low resistive value for the shunt resistor in the first place. As is well known, power consumption of a resistor when current passes through the resistor has the following relationship:
xe2x80x83P=IOUT 2Rxe2x80x83xe2x80x83(3)
where P is the power consumed by the shunt resistor in Watts; R is the ohmic value of the shunt resistor; and IOUT is as defined above.
Very often, to meet the low resistive value R and high power dissipation requirements, the shunt resistor with a large physical size has to be selected. Modern day designs of power converters require compactness where the use of large components is not practical. Further, as shown in equation (3), the relationship between the power consumption P and the current IOUT is not linear. Rather, the power consumption P is proportional the square of the current IOUT passing through the resistor. A small increase in current always results in a significant increase in power dissipation.
Furthermore, as is also known in the art, heat also effects the resistive value of a resistor. Excessive self-generated heat from the shunt resistor may cause the shunt resistor drifting in resistive value and thus may yield inaccurate current reading of the output current IOUT. Sophisticated thermal management or temperature compensation circuitry may be implemented to rectify such shortfalls but it surely will result in high manufacturing cost and design complication.
U.S. Pat. No. 6,366,484, entitled xe2x80x9cCross-Current Sensing in Power Conversion,xe2x80x9d issued to Jin on Apr. 2, 2002, addresses the aforementioned problem and discloses an arrangement which senses current in the primary circuit for the current monitoring of the secondary circuit. U.S. Pat. No. 6,366,484, commonly assigned to the present assignee and incorporated herein by reference in its entirely, teaches the use of current-to-voltage converter to convert the sensed primary current into a corresponding voltage, which in turn is maintained by a sample-and-hold circuit for measurement. The present invention involves direct measurement of the sensed primary current via a RC (resistance-capacitance) circuit. The present invention is capable of not only reporting the average reading of the secondary current but also the instantaneous value of the secondary current.
Electronic circuits are now built with ever-increasing miniaturization and complexity. These circuits must be driven by power converters with high reliability achieved in part by sensing current accurately for the delivery of correct power levels. Without resorting to costly and complex designs but with ease in implementation, there is a need to provide precise schemes in sensing output current of DC-to-DC power converters.
It is accordingly the object of the invention to provide a DC-to-DC power converter with a current sensing mechanism having relative ease and simplicity in implementation. It is also another object of the invention to provide such a converter at low cost and high operational reliability.
The DC-to-DC power converter in accordance with the invention includes a transformer disposed between an input circuit and an output circuit. The transformer has primary and secondary windings coupled to the respective input and output circuits. Current passing through the input circuit is sensed and detected by a detecting circuit which provides the sensed current to a RC circuit which in turn generates a signal proportional in magnitude to the output current sourcing out of the output circuit. If the converter is a step-down converter, the output current is higher than the input current. As arranged, sensing and monitoring the input current instead of the higher output current allows simpler circuit design, lower cost and higher operational reliability.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which like reference numerals refer to like parts.